M. Teresa Vieira

Physicochemical characterization and in vitro dissolution behavior of nicardipine-cyclodextrins inclusion compounds.

Inclusion complexation between nicardipine hydrochloride (NC), a calcium-channel antagonist, and beta-cyclodextrin (beta-CD) or hydroxypropyl-beta-cyclodextrin (HPbetaCD) was evaluated in aqueous environment and in solid state. The phase solubility profiles with both cyclodextrins (CDs) were classified as A(L)-type, indicating the formation of 1:1 stoichiometric inclusion complexes. Stability constants (Ks) were calculated from the phase solubility diagrams and were found to be pH dependent. More stable NC:CDs complexes were formed in alkaline medium in which the drug is in its non-ionized form. Binary systems of NC with CDs, prepared experimentally by different techniques (kneading, evaporation, freeze-drying and spray-drying), were investigated by differential scanning calorimetry, Fourier transformation-infrared spectroscopy, X-ray diffractometry and scanning electron microscopy. From this analysis, evaporation, freeze-drying and spray-drying were found to produce inclusion complexes. In contrast, crystalline drug was still clearly detectable in the kneaded products. The dissolution profiles of the obtained powders were studied in order to define the most appropriate CD and preparation method to originate inclusion complexes, which will be used in the development of a new controlled release formulation of NC. Both the preparation and nature of carrier played an important role in the dissolution performance of the system. However, independently of the preparation technique, all the combinations with HPbetaCD were more effective in achieving the enhancement of the NC dissolution rate, yielding better performances than the corresponding ones with betaCD.

Joining of Superalloys to Intermetallics Using Nanolayers

Joining nickel based superalloys to gamma-TiAl intermetallic alloys will contribute to a more efficient application of these advanced materials, particularly in extreme environments. In this study, Inconel alloy and gamma-TiAl are joined using as filler alternated nanolayer thin films deposited onto each base material. The nanolayers consisted in Ni/Al exothermic reactive multilayer thin films with periods of 5 and 14 nm deposited by d.c. magnetron sputtering in order to improve the adhesion to the substrates and to avoid the reaction between Ni and Al. Diffusion bonding experiments with multilayer coated alloys were performed under vacuum at 800°C by applying 50 MPa during 1h. Bonding was achieved in large areas of the centre of the joints where regions without cracks or pores were produced, especially when using multilayer thin films with a 14 nm modulation period.

Reaction-Assisted Diffusion Bonding of Advanced Materials

The aim of this work is to join -TiAl intermetallics to Ni based superalloys by solid state diffusion bonding. The surface of the -TiAl alloys and Ni superalloys to be joined was prepared by magnetron sputtering with a few microns thick Ni/Al reactive multilayer thin films with nanometric modulation periods. Sound joining without cracks or pores is achieved along the central region of the bond, especially at 800°C and when a 14 nm period Ni/Al film is used as filler material. During the diffusion bonding experiments interdiffusion and reaction inside the Ni/Al multilayer thin film and between the interlayer film and the base materials is promoted with the formation of intermetallic phases. The final reaction product in the multilayer films is the B2-NiAl intermetallic phase. The interfacial diffusion layers between the base materials and the multilayer films should correspond to: 3-NiTiAl and 4-Ni2TiAl phases from the -TiAl side; Ni-rich aluminide and -phase from the Inconel side. These intermetallic phases are responsible for the hardness increase observed on the diffusion layers.

Effect of the Ni chemical distribution on the reactivity and densification of WC-(Fe/Ni/Cr) composite powders

The objective of this work was to study the effect of the Ni distribution on the reactivity and densification of WC-(Fe/Ni/Cr) composite powders. For such, stainless steel AISI 304, was used as a binder base composition which was enriched with Ni by three different processing methods: WC sputter deposition using a target of stainless steel with Ni discs, conventional wet milling of commercial powders (WC, stainless steel and Ni powders) and a previous coating of the WC particles with Ni, followed by the conventional mixing of this coated powder with stainless steel powder. The reactive sintering of these composite powders with identical compositions was investigated. The powder compacts were characterized by scanning electron microscopy and X-ray diffraction with Rietveld analysis to quantify de crystalline phases present.

Study of Sintering Variables of Tungsten Carbide Particles Sputter-Deposited with Stainless Steel

WC-stainless steel 304 (AISI) composite powders have been prepared by an innovative process, which consists in the use of a magnetron sputtering to coat WC powder particles with the stainless steel elements. The characteristics of the particle surfaces are strongly changed by the sputtering process, leading to rough surfaces with increased reactivity. The objective of this work was to study the sintering variables of these tungsten carbide-stainless steel powders. For such, powder compacts were sintered using different conditions of thermal cycle. Samples were characterised by inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction and scanning electron microscopy with chemical analysis. The experimental results are analysed and the sintering conditions leading to densified compacts with a controlled composition, microstructure and crystallographic structure are discussed.

Manufacture of Ceramic Products Using Inertized Aluminum Sludges

The aim of the present research work was to study the effect of mixing aluminum hydroxide sludges with wastes resulting from the cutting and polishing of dimensional stones, which have high content of alumino-silicates, in order to develop mullite. This study shows that, after sintering different mixtures to temperatures up to 1300oC, there is an important increase of secondary mullite phase in the resultant material, particularly in the mixes 2:1 (alumina:silica ratio) simultaneously with a significant decrease of silica phases. The presence of important quantities of -alumina was also detected with the increasing of sintering temperature. Therefore the product obtained after sintering was a composite of mullite and alumina. The mullitisation behaviour was studied using X-ray diffraction and microstructural analysis, which have confirmed the increase of mullite attaining a maximum when the sintering temperature was 1270oC. The composite formed during sintering was responsible for a flexure modulus higher than 100 MPa, with a Weibull Modulus typical of technical ceramics, without degrading other properties, like water absorption. The new developed material was found to be inert after leaching tests carried out according to DIN standard 38414.

Characterisation of Modified Sputtered (TiAl)-Based Intermetallic Materials Doped with Silver and Chromium

TiAl-M (M = Ag, Cr) thin films were synthesised by sputtering and heat-treated in order to obtain the γ-TiAl phase. The results showed that the addition of silver or chromium does not lead to a significant improvement of the mechanical properties of the γ-TiAl structure. However, structural results obtained from similar materials produced by foundry confirm that sputtering might be used as a screening technique, in order to give helpful information for the future production of better performance bulk (titanium aluminide)-based materials. INTRODUCTION Different methodologies have been followed with a view to attempting to increase the ductility of the ordered phase γ-TiAl: (i) the use of the composition Ti-48Al, as a way of obtaining a two-phase structure formed by γ-TiAl together with a small amount of α2-Ti3Al [1,2] (ii) microstructural refinement by heat treatment [3], (iii) the addition of alloying elements in order to reduce the covalent degree of Ti-Al bonding [4-6] and finally (iv) the use of new processing techniques, such as mechanical alloying [7], rapid quenching [8] and sputtering followed by posterior annealing, to obtain the ordered γ-phase with controlled grain size [9-11]. The aim of this study is to use magnetron sputtering as a screening technique, to produce modified titanium aluminides with different contents of silver and chromium, which could be produced later as bulk materials using other techniques. EXPERIMENTAL DETAILS Ti-xAl (0≤x≤100at.%) and TiAl-xM (M=Ag, Cr and 0≤x≤10at.%) thin films (table I) with ≈ 3μm thickness were co-deposited by d.c. magnetron sputtering onto metallic substrates. The Ti-xAl films were obtained from a single target with foils of the other metal superimposed (titanium target with aluminium foils or vice-versa). The TiAl-M films were sputtered from two elemental targets – aluminium and titanium – onto which silver and chromium foils of differing sizes were placed. The samples were studied in their as-deposited state and after isothermal annealing at high temperatures and holding times in a hydrogenated argon atmosphere (5% H2). The chemical composition of the films was determined by electron probe microanalysis (EPMA). X-ray diffraction experiments were performed with Co-Kα radiation. Differential scanning calorimetry (DSC) measurements were carried out in a dynamic N2 atmosphere with 5% of H2. Films for TEM analysis were thinned on both sides by ion milling in an argon atmosphere. A 300kV TEM equipment was used. The hardness tests were carried out with loads of 70 and 300mN in an ultramicroindentation device with a Vickers indenter. A correction method [12] was applied during the calculation procedure. Table I – Chemical composition (at.%) of the (a) Ti-xAl and (b) TiAl-xM (M=Ag, Cr) thin films (a) (b) The ductility of the films was evaluated using a tensile sample with a geometry developed for this purpose [13]. The deformation gradient in the sample was determined measuring it in fifteen regions along the sample (each one 5mm long). The deformation was measured using a travelling microscope, with a accuracy of 1μm. After tensile test the films present cracks where the strain imposed exceeds their ductility, as seen in figure 1. Optical microscopy was used to define the boundary of the region where cracks appear. The strain attained is this region characterises the ductility of the film. Figure 1 Example of observations, by optical microscopy, after deformation, showing the development of cracks. Three different regions of a TiAl-Ag sample, annealed during 1 h, are shown: (a) 1%, (b) 2% and (c) 5% of deformation. (b) (a)

Joining of Gamma-Based Titanium Aluminides – A Review

The optimisation of joining technologies is essential to the application of advanced materials in the design of parts and devices. The development of intermetallic compounds, as structural materials, inevitably requires a new approach to join these compounds to themselves or to other materials. Among different intermetallic classes, titanium aluminides are one of the most studied. However, the industrial application is far from being proportional to the research, due to different problems, where joining processes have an important role. The present paper highlights the state of art on joining γ-TiAl alloys. A review is presented with special emphasis on solid-state diffusion bonding process, because it seems to be the most suitable technique to produce high quality joints of advanced materials. The influence of the bonding conditions on the physical and mechanical properties of the joints is highlighted and the introduction of single or multiple interlayers to assist in the bonding process is discussed. A novel approach developed by the authors to the solid-state diffusion bonding of γ-TiAl alloys using Ti/Al multilayer thin films as bonding materials is proposed. The improvement of the solid-state diffusion bonding will induce sound joints at lower temperatures or pressures.

Joining of TiAl using a thin multilayer

As TiAl based alloys begin to approach maturity, the development of successful and cost effective joining methods will be required. The growing industrial interest in these materials, particularly in aerospace and automotive industry, led to an interesting challenge - how to joint parts and components in order to produce integrated and resistant structures. Diffusion bonding of materials produces components with thinner interfaces than other joining techniques do. The absence of abrupt microstructure discontinuity and the small deformation induced maximize joint strength. This work focuses on the joining of TiAl using a thin multilayer obtained by alternating nanometric layers of titanium and aluminium. The Ti/Al layers were deposited onto the γ-TiAl samples by DC magnetron sputtering. The interfaces of these diffusion bonded joints depend on processing and deposition conditions. In this work we describe the influence of bilayer thickness (period) and on microstructure and chemical composition of the joining interfaces.

Improving the Cutting Performance of TiAlN Coatings Using Submicron Metal Interlayers

TiAlN coatings as been used with success in high speed metal machining. The similar B1 TiN structure, high hardness, adhesion and high wear and oxidation resistance contributes to such ability. However, it is possible to enhance some of those properties, particularly the adhesion and cohesion of the coatings without changing significantly all the others. The introduction of submicron metal interlayers in TiAlN coatings do not affect significantly the hardness of the whole coating itself, but promote a beneficial decrease in the internal stresses and an increase in the film adhesion and cohesion which contributes to improve their performance in service. Moreover, the possibility to form morphological discontinuities in the coating due to the presence of metal interlayers can enhance diffusion barriers. So, in the present work, multilayer coatings of TiAlN with Al and Ti interlayers with approximately 3.5μm thick were sputtered. The internal stress, hardness, Young modulus and adhesion of the coatings were evaluated as a function of number of layers.